Preparation and performance evaluation of a nano-enhanced miscibility system for CO 2 miscible flooding in medium-viscosity sandstone reservoirs

To address the challenges encountered in CO2 miscible flooding in Block N, a nano-enhanced miscibility system was constructed by first formulating a miscibility-enhancing base fluid using a PVAy series miscibility enhancer compounded with cosolvents, followed by the introduction of nanoparticles, and its performance was systematically evaluated. The results showed that among the PVAy series, PVA20 exhibited the best compatibility with the crude oil from Block N, reducing the CO2-oil interfacial tension by 75.3 ± 0.10%. Further compounding with BCS, Span80, and ethyl alcohol revealed that the miscibility-enhancing base fluid with a PVA20 to BCS mass ratio of 1:1 achieved the optimal solubilization and viscosity reduction. On this basis, SiO2 nanoparticles were introduced to obtain the final optimized nano-enhanced miscibility system: 0.2 wt% PVA20 + 0.2 wt% BCS + 0.25 wt% SiO2. This system demonstrated good dispersion stability, reducing the minimum miscibility pressure between CO2 and Block N crude oil from 30.61 MPa to 24.51 MPa and increasing the asphaltene precipitation onset point by 10 percentage points. CO2 core flooding experiments demonstrated that this system enhanced oil recovery by 10.87 percentage points and promoted the lightening of produced oil, decreasing the C30+ fraction from 10.36% to 7.16%. This study provides experimental evidence and technical reference for the field application of CO2 miscible flooding in medium-viscosity sandstone reservoirs.